Self-regulating, electrically semi-conductive, heating cables are well known in the art. They generally feature at least one pair of elongate electrical conductors such as copper wires that are spaced apart from each coextensively along the length of the cable within a semi-conductive composition that typically comprises one or more polymeric materials such as a polyolefin or fluorocarbon or chlorofluorocarbon materials containing uniformly dispersed electrically conductive particles, commonly conductive carbon black particles, enclosed within at least one electrically insulative protective jacket. Examples of self-regulating heating cables of the type described above are disclosed for example in U.S. Pat. Nos. 3,858,144; 4,188,276; 4,200,973; 4,277,673; 4,327,480; 4,334,351 and 4,334,148, the disclosures of which are incorporated herein by reference.
Typically, the semi-conductive composition is a composition that exhibits a positive coefficient of electrical resistance with temperature and possesses sufficient crystallinity to promote the self-regulating characteristics desired. Generally, it is common practice to anneal such cables at a temperature at or above the melting temperature of the crystalline portion of the semi-conductive composition for a period of time sufficient to stabilize the electrical resistance and then crosslink the composition, commonly by radiation, to promote the self-regulating characteristics of the composition.
In order to maintain a precise distance between the electrical conductors during the annealing step, it has been the practice to enclose the conductors and semi-conductive compositions with an electrically insulating shape-retaining jacket that is made from a material that has a softening temperature that is higher than the temperature at which the semi-conductive composition is to be annealed. Such cables also typically include one or more polymeric and/or metallic jackets about the shape-retaining jacket for added mechanical protection and the like.
The semi-conductive compositions used in self-regulating heating cables of the type described above typically contain from about 4% to about 25% by weight to the total weight of the composition of one or more electrically conductive carbon black types to promote the self-regulating heating characteristics of the cable. Depending upon the amount of carbon black or other suitable conductive particles present in the semi-conductive composition, such cables will exhibit certain electrical resistance at ambient temperature and a certain profile of increasing electrical resistance for increasing temperatures as is associated with positive temperature coefficient materials as is well known to those ordinarily skilled in the art.
One of the problems in producing such cables is the maintenance of a close tolerance range on ambient electrical resistance of the semi-conductive composition from batch to batch and the stability of the ambient electrical resistance with time and temperature. Although the previously described crosslinking and annealing practice can be used to advantage in controlling the electrical resistance characteristics, it has been discovered that one of the problems contributing to variations in electrical resistance of such semi-conductive compositions is the nature of the jacket used to enclose the semi-conductive composition.
Heretofore, it was thought that any electrically insulative material that was compatible with the semi-conductive composition and possessed a softening temperature higher than the melting temperature of the crystalline portion of the semi-conductive composition in combination with suitable strength characteristics would be suitable. Typically, such jackets in the past have been made from a thermoplastic polyurethane, polyamide or polyester material as described for example in the previously described patents. Polyvinyl chloride materials have also been used for such cables as a means of providing flame retardancy. It has been discovered that self-regulating heating cables that utilize a flexible jacket made from a material containing a plasticizer such as polyvinyl chloride suffer a substantial and undesirable increase in ambient electrical resistance over a period of time and that such will often be the case even when an intermediate jacket made from an unplasticized material is interposed between the semi-conductive composition and the plasticizer-containing jacket and that such is particularly a problem where it is desired to impart flame retardancy to the cable by enclosing the cable with a flame retardant jacket made from a halogenated material such as polyvinyl chloride.
It has also been found that jackets made from conventional materials such as polyurethane or polyester or polyamide suffer marked color change upon exposure to radiation which understandably is a problem in providing jackets of one or more predetermined colors for use in various applications for radiation is commonly used as a method of crosslinking such semi-conductive compositions.
In view of the above, a need exists for providing a protective jacket about self-regulating, electrically semi-conductive heating cables that is able to minimize changes in the ambient electrical resistance of the semi-conductive material arising from exposing the cable to elevated operating temperature conditions and is further able to incorporate halogenated flame retardants without the need for plasticizers when it is desired to impart flame retardancy to the cable and is additionally able to provide improved resistance to color change as well as to degradation arising from exposure to radiation and elevated annealing temperatures associated with the process of making the cable.